The gzip utility reduces the size of the named
files using adaptive Lempel-Ziv coding, in deflate mode. If invoked as
gzip -O, the compress mode of compression is
chosen; see
compress(1) for
more information. Each file is renamed to the same name plus the extension
“.gz”. As many of the modification time, access time, file
flags, file mode, user ID, and group ID as allowed by permissions are retained
in the new file. If compression would not reduce the size of a file, the file
is ignored (unless -f is used).

The gunzip utility restores compressed files to
their original form, renaming the files by removing the extension (or by using
the stored name if the -N flag is specified). It
has the ability to restore files compressed by both
gzip and
compress(1),
recognising the following extensions: “.Z”, “-Z”,
“_Z”, “.gz”, “-gz”,
“_gz”, “.tgz”, “-tgz”,
“_tgz”, “.taz”, “-taz”, and
“_taz”. Extensions ending in “tgz” and
“taz” are not removed when decompressing, instead they are
converted to “tar”.

The gzcat command is equivalent in functionality to
gunzip-c.

If renaming the files would cause files to be overwritten and the standard input
device is a terminal, the user is prompted (on the standard error output) for
confirmation. If prompting is not possible or confirmation is not received,
the files are not overwritten.

If no files are specified, the standard input is compressed or uncompressed to
the standard output. If either the input or output files are not regular
files, the checks for reduction in size and file overwriting are not
performed, the input file is not removed, and the attributes of the input file
are not retained.

By default, when compressing, the original file name and time stamp are stored
in the compressed file. When uncompressing, this information is not used.
Instead, the uncompressed file inherits the time stamp of the compressed
version and the uncompressed file name is generated from the name of the
compressed file as described above. These defaults may be overridden by the
-N and -n flags,
described below.

Use the deflate scheme, with compression factor of
-1 to -9.
Compression factor -1 is the fastest, but
provides a poorer level of compression. Compression factor
-9 provides the best level of compression,
but is relatively slow. The default is
-6.

Force compression of file,
even if it is not actually reduced in size. Additionally, files are
overwritten without prompting for confirmation. If the input data is not
in a format recognized by gzip and if the
option -c is also given, copy the input data
without change to the standard output: let
gzcat behave as
cat(1).

gzip uses a modified Lempel-Ziv algorithm (LZW).
Common substrings are replaced by pointers to previous strings, and are found
using a hash table. Unique substrings are emitted as a string of literal
bytes, and compressed as Huffman trees. When code 512 is reached, the
algorithm switches to 10-bit codes and continues to use more bits until the
limit specified by the -b flag is reached.
bits must be between 9 and 16 (the default is
16).

After the bits limit is reached,
gzip periodically checks the compression ratio.
If it is increasing, gzip continues to use the
existing code dictionary. However, if the compression ratio decreases,
gzip discards the table of substrings and
rebuilds it from scratch. This allows the algorithm to adapt to the next
“block” of the file.

The -b flag is omitted for
gunzip since the
bits parameter specified during compression
is encoded within the output, along with a magic number to ensure that neither
decompression of random data nor recompression of compressed data is
attempted.

The amount of compression obtained depends on the size of the input, the number
of bits per code, and the distribution of
common substrings. Typically, text such as source code or English is reduced
by 60 - 70% using gzip. Compression is generally
much better than that achieved by Huffman coding (as used in the historical
command pack), or adaptive Huffman coding (as used in the historical command
compact), and takes less time to compute.